JAVA多线程之中断机制stop()、interrupted()、isInterrupted()

JAVA多线程之中断机制stop()、interrupted()、isInterrupted()

一，介绍

本文记录java多线程中的中断机制的一些知识点。主要是stop方法、interrupted()与isInterrupted()方法的区别，并从源代码的实现上进行简单分析。

JAVA中有3种方式可以终止正在运行的线程

①线程正常退出，即run()方法执行完毕了

②使用Thread类中的stop()方法强行终止线程。但stop()方法已经过期了，不推荐使用

③使用中断机制

线程正常退出没有什么东东，中断机制下面详细介绍，先看下stop()方法的源代码，关键是源代码上的注释。它解释了为什么stop()不安全，stop()方法停止的是哪个线程？

/**

* Forces the thread to stop executing.

*

* If there is a security manager installed, ihttp://ts checkAccess

* method is called with this

* as its argument. This may result in a

* SecurityException being raised (in the current thread).

*

* If this thread is different from the current thread (that is, the current

* thread is trying to stop a thread other than itself), the

* security manager's checkPermission method (with a

* RuntimePermission("stopThread") argument) is called in

* addition.

* Again, this may result in throwing a

* SecurityException (in the current thread).

*

* The thread represented by this thread is forced to stop whatever

* it is doing abnormally and to throw a newly created

* ThreadDeath object as an exception.

*

* It is permitted to stop a thread that has not yet been started.

* If the thread is eventually started, it immediately terminates.

*

* An application should not normally try to catch

* ThreadDeath unless it must do some extraordinary

* cleanup operation (note that the throwing of

* ThreadDeath causes finally clauses of

* try statements to be executed before the thread

* officially dies). If a catch clause catches a

* ThreadDeath object, it is important to rethrow the

* object so that the thread actually dies.

*

* The top-level error handler that reacts to otherwise uncaught

* exceptions does not print out a message or otherwise notify the

* application if the uncaught exception is an instance of

* ThreadDeath.

*

* @exception SecurityException if the current thread cannot

* modify this thread.

* @see #interrupt()

* @see #checkAccess()

* @see #run()

* @see #start()

* @see ThreadDeath

* @see ThreadGroup#uncaughtException(Thread,Throwable)

* @see SecurityManager#checkAccess(Thread)

* @see SecurityManager#checkPermission

* @deprecated This method is inherently unsafe. Stopping a thread with

* Thread.stop causes it to unlock all of the monitors that it

* has locked (as a natural consequence of the unchecked

* ThreadDeath exception propagating up the stack). If

* any of the objects previously protected by these monitors were in

* an inconsistent state, the damaged objects become visible to

* other threads, potentially resulting in arbitrary behavior. Many

* uses of stop should be replaced by code that simply

* modifies some variable to indicate that the target thread should

* stop running. The target thread should check this variable

* regularly, and return from its run method in an orderly fashion

* if the variable indicates that it is to stop running. If the

* target thread waits for long periods (on a condition variable,

* for example), the interrupt method should be used to

* interrupt the wait.

* For more information, see

* Why

* are Thread.stop, Thread.suspend and Thread.resume Deprecated?.

*/

@Deprecated

public final void stop() {

stop(new ThreadDeath());

}

上面注释，第9行到第16行表明，stop()方法可以停止“其他线程”。执行thread.stop()方法这条语句的线程称为当前线程，而“其他线程”则是 调用thread.stop()方法的对象thread所代表的线程。

如：

public static void main(String[] args) {

MyThread thread = new MyThread...

//.....

thread.stop();

//....

}

在main方法中，当前线程就是main线程。它执行到第4行，想把“其他线程”thread“ 给停止。这个其他线程就是MyThread类 new 的thread对象所表示的线程。

第21行至23行表明，可以停止一个尚未started(启动)的线程。它的效果是：当该线程启动后，就立马结束了。

第48行以后的注释，则深刻表明了为什么stop()方法被弃用！为什么它是不安全的。

比如说，threadA线程拥有了监视器，这些监视器负责保护某些临界资源，比如说银行的转账的金额。当正在转账过程中，main线程调用 threadA.stop()方法。结果导致监视器被释放，其保护的资源（转账金额）很可能出现不一致性。比如，A账户减少了100，而B账户却没有增加100

二，中断机制

JAVA中如何正确地使用中断机制的细节太多了。interrupted()方法与 isInterrupted()方法都是反映当前线程的是否处于中断状态的。

①interrupted()

/**

* Tests whether the current thread has been interrupted. The

* interrupted status of the thread is cleared by this method. In

* other words, if this method were to be called twice in succession, the

* second call would return false (unless the current thread were

* interrupted again, after the first call had cleared its interrupted

* status and before the second call had examined it).

*

*

A thread interruption ignored because a thread was not alive

* at the time of the interrupt will be reflected by this method

* returning false.

*

* @return true if the current thread has been interrupted;

* false otherwise.

* @see #isInterrupted()

* @revised .

*/

public static boolean interrupted() {

return currentThread().isInterrupted(true);

}

从源码的注释中看出，它测试的是当前线程(current thread)的中断状态，且这个方法会清除中断状态。

②isInterrupted()

/**

* Tests whether this thread has been interrupted. The interrupted

* status of the thread is unaffected by this method.

*

*

A thread interruption ignored because a thread was not alive

* at the time of the interrupt will be reflected by this method

* returning false.

*

* @return true if this thread has been interrupted;

* false otherwise.

* @see #interrupted()

* @revised .

*/

public boolean isInterrupted() {

return isInterrupted(false);

}

从源码注释中可以看出，isInterrupted()方法不会清除中断状态。

③interrupted()方法与 isInterrupted()方法的区别

从源代码可以看出，这两个方法都是调用的isInterrupted(boolean ClearInterrupted),只不过一个带的参数是true，另一个带的参数是false。

/**

* Tests if some Thread has been interrupted. The interrupted state

* is reset or not based on the value of ClearInterrupted that is

* passed.

*/

private native boolean isInterrupted(boolean ClearInterrupted);

因此，第一个区别就是，一个会清除中断标识位，另一个不会清除中断标识位。

再分析源码，就可以看出第二个区别在return 语句上：

public static boolean interrupted() {

return currentThread().isInterrupted(true);

}

/************************/

public boolean isInterrupted() {

return isInterrupted(false);

}

interrupted()测试的是当前的线程的中断状态。而isInterrupted()测试的是调用该方法的对象所表示的线程。一个是静态方法（它测试的是当前线程的中断状态），一个是实例方法（它测试的是实例对象所表示的线程的中断状态）。

下面用个具体的例子来更进一步地阐明这个区别。

有一个自定义的线程类如下：

public class MyThread extends Thread {

@Override

public void run() {

super.run();

for (int i = ; i < ; i++) {

System.out.println("i=" + (i + ));

}

}

}

先看interrupted()方法的示例：

public class Run {

public static void main(String[] args) {

try {

MyThread thread = new MyThread();

thread.start();

Thread.sleep();

thread.interrupt();

//Thread.currentThread().interrupt();

System.out.println("是否停止？="+thread.interrupted());//false

System.out.println("是否停止？="+thread.interrupted());//false main线程没有被中断!!!

　　　　　　//......

第5行启动thread线程，第6行使main线程睡眠1秒钟从而使得thread线程有机会获得CPU执行。

main线程睡眠1s钟后，恢复执行到第7行，请求中断 thread线程。

第9行测试线程是否处于中断状态，这里测试的是哪个线程呢？？？答案是main线程。因为：

（1）interrupted()测试的是当前的线程的中断状态

（2）main线程执行了第9行语句，故main线程是当前线LwOkEcY程

再看isInterrupted()方法的示例：

public class Run {

public static void main(String[] args) {

try {

MyThread thread = new MyThread();

thread.start();

Thread.sleep();

thread.interrupt();

System.out.println("是否停止？="+thread.isInterrupted());//true

在第8行，是thread对象调用的isInterrupted()方法。因此，测试的是thread对象所代表的线程的中断状态。由于在第7行，main线程请求中断 thread线程，故在第8行的结果为: true

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